Method for producing a tomographic image of the body and electric impedance tomograph

ABSTRACT

The invention is a method of obtaining tomographic images of the human body and the electrical impedance tomograph, in which a source of electric current is used to send electric current at levels undetectable by a human being to pairs of electrodes, between which at least two electrodes are placed. An algorithm of image reconstruction makes it possible to obtain the distribution of absolute conductivity of a body, characterizing the state of soft and bone tissues and blood vessels. The method is fast. It allows one to visualize the changes of conductivity during one cardiocycle and to observe blood filling the heart and vessels. It allows one to obtain time dependence of conductivity of internal areas of the heart, which is an impedance cardiogram, containing additional information about heart function. The use of visualization of conductivity of tissues allows one to observe the processes of internal hemorrhages, to reveal inflammations, to carry out studies of organs of digestion, to observe the state of various tumors, to carry out diagnosis of diseases of the mammary gland, to diagnose various lung diseases. The method allows one to monitor the variation of temperature of internal organs, raising the possibility of diagnosing diseases at early stages. The tomograph is a rather simple, compact device, convenient in operation, safe for a patient and attendants, operating with a standard personal computer. This device can be widely used in medical practice and clinical investigations.

Applicants claim priority based on international applicationPCT/RU97/00398 filed Dec. 5, 1997 and designating the United States ofAmerica.

FIELD OF TECHNOLOGY

This invention provides methods for medical diagnosis using a device toobtain tomographic images of a patient's body.

The device is simple, compact, and convenient to use. It allowstomographic studies to be performed quickly, which is less fatiguing tothe patient. The system is safe for the patient and the operator of thedevice. The images obtained characterize the state of soft and bonetissues and blood vessels. The device allows one to visualize changes inthe conductivity of tissues rapidly, for example during one cardiocycle.One can observe blood filling the heart and vessels. The device candetermine various characteristics of an organism's state, in particularthe time dependence of conductivity of any area of the heart. This iscalled an impedance cardiogram. The ability to visualize theconductivity of tissues allows one to observe the processes of internalhemorrhages and digestive organs, to study the state of the lungs, todetect various tumors, and to monitor the variation of temperature ofinternal organs. These capabilities give one the ability to diagnosemany diseases at their earliest stages.

BACKGROUND OF THE INVENTION

There are existing methods of obtaining tomographic images of a humanbody based on the measurement of the spatial distribution of a physicalfield or radiation that penetrates the object and subsequentreconstruction of the image using the spatial distribution of measuredparameters and mathematical methods of convolution and back projection.(The Physics of Medical Imaging. Edited by Steve Webb. Adam Hilger,Bristol and Philadelphia. Chapter 8).

Tomographs, based on the use of x-ray radiation or nuclear magneticresonance (NMR), are known. (The Physics of Medical Imaging. Edited bySteve Webb. Adam Hilger, Bristol and Philadelphia. Chapter 8).

The known tomographic methods provide high resolution. However, thecomplicated x-ray or NMR setups used for diagnostics are expensive anddifficult in operation, the procedure of inspection is rather long, andthe radiation, penetrating a body, is not harmless for patients andoperators.

The method of obtaining of a tomographic image of a human body formedical diagnostics, based on the use of electric current, is known aselectrical impedance tomography. (Patent of Great Britain 2119520 A, INTCL A61B 5/05, 1983). In the known method a series of contact electrodesis placed on the surface of a patient's body; a source of electriccurrent is connected sequentially to pairs of electrodes; measurementsof potential differences (voltages) between pairs of electrodes, arisingbecause of the current flow through the object, are made. Referencevalues of potential differences are determined based on the assumptionof homogeneity of electrical conductivity of the object, or by measuringthe same object at different times if the electrical conductivitychanges. An image is constructed—from the spatial conductivitydistribution of a body or from changes in the conductivity between twomeasurements—using back projection or the relative differences ofmeasured and reference voltages along equipotential lines of an electricfield. It is established that the electrical conductivity of biologicaltissue depends on its physiological properties. The conductivitydistribution of a body can be used to create images of bones, softtissues and blood vessels.

The electrical impedance tomograph consists of a system of contactelectrodes, a unit for electric current excitation, a unit formeasurement of potential differences, a microprocessor-based controlcircuit, a differential amplifier, and analog multiplexers, the inputsof which are connected to contact electrodes and the outputs to theinput of the differential amplifier (Patent of Great Britain 2119520 A,INT CL: A61B 5/05, 1983).

However, the use of the method in clinical practice has been hindereduntil now by the unsolved problem of obtaining absolute or “static”images of satisfactory quality when measurements are carried out on ahuman body. Existing tomographs allow only dynamic tomograms to beobtained, representing images of conductivity changes between twomeasurements, which are not informative for medical applications. Theinability to visualize static objects is due to the inability tocompletely solve the inverse problem of the conductivity reconstructiondue to the difficulty of obtaining reference values of potentialdifferences when neither the geometry of the boundary surface of theobject studied nor the location of measuring electrodes on this surfaceare known exactly.

Visualization of the absolute conductivity distribution in thecross-section of a human body with a high rate of data acquisitionbecame possible by using a compact tomograph with control of all of itsmeasuring functions by personal computer. The computer carries outprocessing, visualization and storage of data. (V. A. Cherepenin, A. V.Korjenevsky et al. The Electrical Impedance Tomograph: NewCapabilities.//IX International Conference on Electrical Bio-Impedance,Proceedings.—Heidelberg, 1995, p. 430-433). The method of obtaining atomographic image of a body described in this reference involves: theplacing of a series of contact electrodes on the surface of a body; thesequential dipole connection of an electric current source to pairs ofadjacent electrodes; the measuring of potential differences between eachpair of the rest of the electrodes; the determination of referencevalues of potential differences; and the reconstruction of the image ofspatial distribution of conductivity of a body by back projection ofweighted relative differences of the reference and measured voltagesalong equipotential lines. Reference values of potential differencesu_(r)^(j)(j)

are determined by approximation of the measured distribution ofpotential differences u_(m)^(j)(j)

according to the expression: $\begin{matrix}{{{u_{r}^{i}(j)} = {{c_{1}^{i}{f_{1}^{i}(j)}} + {c_{2}^{i}{f_{2}^{i}(j)}} + c_{3}^{i}}},} & (1)\end{matrix}$

Where:

i—the number of exciting pairs of electrodes;

j—the number of measuring pairs of electrodes; f₁^(i)(j)−

given distribution of voltage between the adjacent electrodes along theboundary of the reference object; f₂^(i)(j)−

signals caused by spurious couplings; and c_(α)^(i)(α = 1, 2, 3)−

approximation coefficients of the measured distribution of potentialdifferences u_(m)^(i)(j).

In the described solution it is possible to construct a reference dataset which does not contain information about the interior structure ofthe object by using an approximation of the measured data u_(m)^(i)(j)

by smooth dependencies from a set of simple linearly independentfunctions. This set, together with an initial set including variationsthat characterize the interior structure of the object, is used forreconstruction of the absolute conductivity of the object. The measuredpotential differences can contain considerable systematic errors causedprimarily by spurious penetration of signals from channel to channel inthe integral multiplexers and input circuits of the tomograph. Duringthe reconstruction of the distribution of spatial conductivity, thesenoises cause the appearance of artifacts and significantly reduce thequality of the image. To eliminate their influence, a set of spurioussignals can be included in the set of base functions used forapproximation of the input data. Best results are obtained by using acombination of three functions mentioned in equation (1). Thedistribution f₁^(i)(j)

is the distribution of voltage between the adjacent electrodes along theboundary of a cylindrical object with homogeneous conductivity when anelectric current source is connected to the pair of adjacent electrodes.

The developed algorithm for the reconstruction of the conductivitydistribution allows one to obtain medically useful and informative“static” images characterizing the physiological state of organs andtissues. However, the low sensitivity and rather low resolution of thismethod limits the range of its application.

SUMMARY OF THE INVENTION

In the present invention the method of obtaining a tomographic image ofa human body is presented. It allows one to obtain qualitativevisualization of conductivity inside a body with high sensitivity andsatisfactory resolution, dynamically characterizing the state ofinternal organs and tissues with high reliability. The method canincrease the signal-to-noise ratio more than one order of magnitude andthus increase the sensitivity and resolution of the tomographic device.The device is suitable for wide spread use in medical practice andclinical investigations. The method presented here reveals and measuresstructures and processes, determination of which is difficult orimpossible by use of x-rays or nuclear magnetic resonance. The methodallows easy and safe diagnosis of a patient not only in clinicalenvironments but also in physicians' offices and in laboratories.

The problem of increasing the quality of tomographic images is solved bythe development of a new effective method for measuring the potentialdifferences and for reconstructing the image of the spatial distributionof conductivity found inside a body.

The invention is the method of obtaining a tomographic image of a body,including the placing of a series of contact electrodes on the surfaceof a human body, connecting the electric current source to pairs ofelectrodes sequentially, measuring the potential difference between eachpair of electrodes, determining the reference values of the potentialdifferences u_(r)^(i)(j)

by the approximation of measured distribution of the potentialdifferences u_(m)^(i)(j)

in accordance with the expressionu_(r)^(i)(j) = c₁^(i)f₁^(i)(j) + c₂^(i)f₂^(i)(j) + c₃^(i)

while the electric current source is connected to those pairs ofelectrodes, between which at least two electrodes are placed, and thereconstruction of the image of spatial distribution of the conductivityof a body by back projection along equipotential lines, made accordingto the following expressions:${S = {\sum{\frac{W^{lt}W^{rt}}{W^{lt} + W^{rt}}( {\lambda^{lt} + \lambda^{rt}} )}}},$

λ^(lt, rt) = u_(r)^(lt, rt)/u_(m)^(lt, rt) − 1, where:

W^(lt), W^(rt)—weight factors determined according to the procedure ofback projection in the direction from the “left” and from the “right”intersection of equipotential lines with the surface of a body,correspondingly,

Σ—a summation over all positions of the injecting electrodes,u_(m)^(lt, rt), −

voltages, measured on the left and on the and right “ends” ofequipotential lines, which pass through the given point of areconstructed cross section, u_(r)^(lt, rt), −

reference potential differences corresponding to a body with homogeneousconductivity,

i—the number of exciting pairs of electrodes,

j—the number of measuring pairs of electrodes,

f_(l)(j)—given distribution of voltage between adjacent electrodes alongthe boundary of a reference object, f₂^(i)(j)−

signals caused by spurious couplings, and c_(α)^(i)(α = 1, 2, 3)−

approximating coefficients of the measured distribution of the potentialdifferences u_(m)^(i)(j).

The best result for increasing the sensitivity of the devices isobtained when the polar injection of electric current by a pair ofdiametrically opposite electrodes is used.

The accuracy of measurements is also increased by carrying outmeasurements on a certain pairs of electrodes while the electric currentsource is connected sequentially to each pair of other electrodes. Theprocedure of connection is then repeated sequentially by makingmeasurements on the next pair of electrodes.

It is efficient to determine the signals f₂^(i)(j),

caused by spurious couplings, by measurement.

For the diagnosis of organ functions when the conductivity varies intime, a series of measurements of the potential differences isautomatically made sequentially in time, a spectral Fouriertransformation of time dependencies of the measured results is carriedout, and the reconstruction of the images of the spatial distribution ofconductivity of the organs for each frequency component is made.

To obtain an electrical impedance cardiogram, a series of measurementsof conductivity of various areas of a heart is made, and the timedependence of this conductivity is determined.

The reconstruction of the image of spatial distribution of absoluteconductivity of a body is carried out by normalizing the measuredconductivity values on the assumption that the least value ofconductivity corresponds to the conductivity of bone tissues and thegreatest value of conductivity corresponds to the conductivity of blood.

The invention can be used to diagnose cardio-vascular disease, toidentify internal hemorrhages and tissue inflammation, to detect tumorformation in its early stages, and to determine the temperature ofinternal organs. It can be used to diagnose diseases of bone tissues. Itcan safely and simply diagnose diseases of the mammary gland.

According to the invention, the electrical impedance tomograph consistsof: a system of contact electrodes; a unit for the excitation ofelectric current; a source of constant voltage; a unit for measuringpotential differences; a microprocessor-based control circuit; adifferential amplifier; analog multiplexers; a circuit for thecompensation of the common-mode component of voltages on each pair ofelectrodes; a circuit for compensation of contact potential differenceson each electrode; and a circuit for control of the quality of contactswhere the inputs of the analog multiplexers are connected to contactelectrodes, the outputs to the input of the differential amplifier. Acircuit for the compensation of the common-mode component of voltagescan be composed of a feedback circuit containing the operationalamplifier. The output of the common-mode signal of the differentialamplifier is connected to the inverting input of the operationalamplifier, the output of which is connected to the unit for excitationof electric current. The circuit for control of contact quality iscomposed as two comparators, the first inputs of which are connected tothe output of the unit for excitation of current. The second input ofone comparator is connected to the positive output of the constantvoltage source. The second input of another comparator is connected tothe negative output of the constant voltage source. The outputs of thecomparators are connected to the microprocessor based control circuit.

It is expedient to compose the compensation circuit for the contactpotential differences as a feedback circuit, that includes an analogswitch and an integrator connected in series. The analog switch input isconnected to the measuring unit, and the output of the integrator isconnected to the zero correction input of the differential amplifier.

The present invention improves on previous solutions in that an optimalmethod of measurement and a new algorithm of reconstruction ofconductivity distribution have been found, providing a quality imagewith high resolution.

During measurements of a human body the value of injected current islimited to safe levels. Therefore, the value of the measured signals isfound to be rather small. The quality of reconstructed images isseriously influenced by the ratio of the amplitude of the measuredsignals to the value of noises of the device and external electricalnoises on the frequency of the measurements. In the present inventionthe signal-to-noise ratio is increased by more than an order ofmagnitude by the injection of the electric current through pairs ofelectrodes separated by at least two electrodes. This allows increasedsensitivity and, in many cases, increased resolution.

For this more general case, compared with dipole injection, it isnecessary to correct the method of reconstruction of the image of thespatial distribution of conductivity. Initial data consist of thepotential differences between adjacent electrodes when the currentsource is connected to some pair of electrodes which are fixed on thepatient's skin along a closed contour, wrapping around the body. Using Nelectrodes, we have N profiles corresponding to a particular variant ofthe connection to the current source, each containing (N-4) values ofthe potential differences between free pairs of electrodes.

The back projection procedure for the arbitrary method of injection ofcurrent is described in the following way:${S = {\sum{\frac{W^{lt}W^{rt}}{W^{lt} + W^{rt}}( {\lambda^{lt} + \lambda^{rt}} )}}},$

λ^(lt, rt) = u_(r)^(lt, rt)/u_(m)^(lt, rt) − 1,

where:

W^(lt,rt)—weight factors, calculated according to the usual procedure ofback projection in the direction from the “left” and from the “right”intersections of the equipotential lines with the body's boundarycorrespondingly,

Σ—the sum over all positions of the injecting electrodes,u_(m)^(lt, rt)−

voltages, measured on the left and on the right “endpoints” ofequipotential lines, passing through the given point of a reconstructedcross-section, u_(r)^(lt, rt)−

reference potential differences corresponding to a body with homogeneousconductivity.

In the present invention, accuracy of measurements and reconstructedimage quality are improved by the correct choice of the sequence ofcontact switching, within the same strategy of the injection of current,through the reduction of the influence of transitory processes thatarise in the input circuits after switching of the receiver from onepair of electrodes to another, without decreasing the fast operation ofthe device. During measurements, the receiver is switched only aftermeasurements with all possible pairs of injecting electrodes arecompleted. Unlike known systems, where for fixed positions of theinjecting electrodes, sequential switching of the measuring electrodesis made, in the present invention the receiver is connected to the samepair of electrodes for a long time, while frequent switches are made onthe injecting pair. The transient processes are completed early in theseries of measurements, and their influence is reduced significantly,while the same total time is required for a complete set ofmeasurements. For a system with 16 electrodes, the influence of thesetransients by the time measurements on a given pair of electrodes iscompleted is e¹²≈10⁵ times smaller than in all the known solutions tothe problem. Moreover, measurement errors are further decreased by theuse of a circuit to control the quality of contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the following drawings.

FIG. 1 is a block-diagram of the tomograph's measuring system.

FIG. 2 shows the time dependence of the conductivity of the interior ofa heart, which is obtained by measurement with dipole injection, as inpreviously invented solutions.

FIG. 3 shows the same dependence as FIG. 2 using the polar injectionmethod described in this patent application.

FIG. 4 shows the spectrum of this dependence.

FIG. 5 shows the impedance tomogram of the thorax of patient Y.

FIG. 6 shows the spatial distribution of the spectral amplitude ofconductivity in the cross-section of the thorax at the frequencycorresponding to the frequency of heart contractions of patient Y.

FIG. 7 shows the impedance tomogram of the thorax of patient X.

FIG. 8 presents a summarized roentgenogram (X-ray photograph) of thethorax of patient X.

FIG. 9 shows the impedance tomogram of the lower part of the left leg ofpatient Z.

FIG. 10 shows the impedance tomogram of the upper part of the left legof patient Z.

The block-diagram of tomograph (FIG. 1) is coded as follows: contactelectrodes 1-16, generator of pulsing voltage 17, voltage-to-currentconverter 18, comparators 19, triggers 20, pulse phase modulator 21,analog multiplexers 22 and analog multiplexers 23 with leads forconnections to electrodes, differential amplifier 24, common modefeedback amplifier 25, integrator 26, analog switch 27, amplifier stage28, lock-in detector 29, switch 30, integrator 31, switch 32,analog-to-digital converter (ADC) 33, microprocessor 34, control signalgenerator 35.

The operation of the system of data acquisition of the impedancetomograph measures the potentials on the surface of a human body withthe aid of contact electrodes 1-16 upon the injection of a weak electriccurrent. The exciting pair of electrodes is connected to a currentsource; other pairs are used for the measurements of voltages caused bythe flow of the current in the exciting electrodes. This variant of theimpedance tomograph is constructed under a single-channel scheme and isdesigned for operation with 16 electrodes. The duration of eachmeasurement is 350 μS, and the frequency of fulfifllment of completecycles of measurements is 11 frames per second. Measurements are carriedout upon the occurrence of a pulsing signal of a special form having anaverage frequency of 8 kHz. The use of this signal simplifies theequipment and permits higher accuracy and faster operation compared topreviously existing systems that use sinusoidal excitation.

The excitation circuit contains a high-precision generator of pulsedvoltage 17, a voltage-to-current converter 18 and a pulse phasemodulator 21. The generator 17 produces pulsed voltage in the form oftwo periods of meander, shifted on 180°, with controlled amplitude whichis established by an 8-bit code from the microprocessor unit 34. The useof this pulsed current in the tomograph eliminates the need for ahigh-precision sinusoidal signal generator. The voltage-to-currentconverter 18 is based on two operational amplifiers. It has a stablegain coefficient, high output resistance, and it provides output currentvalues that are independent of the resistance of the circuit of excitingelectrodes. During measurements, the quality of the electrical contactbetween the electrodes and skin can become poor. This leads to errors inthe measurements arising from disturbance in operation of the currentsource, when the maximum permissible output voltage is reached, as wellas from increasing noise and errors in the input circuits of thereceiver. Therefore, it is important to be able to control the qualityof contacts between electrodes and skin during measurements and todetermnine which contact shows increased electrical resistance and,therefore, requires intervention. To achieve this, two comparators 19are introduced in the circuit of the device, which compare the outputvoltage of the voltageto-current converter with given constant positive+U and negative −U voltages, which are equal to the maximum allowablepositive and negative output voltage of the current source. The outputsignal of comparators 19 corresponding to voltages falling outside ofthe allowable limits is transmitted to the microprocessor block 34 forfurther processing. The output signal from comparators 19, correspondingto the deviation of the voltage from allowable limits, is received bythe mnicroprocessor and, together with the current addresses of theactive contacts, is transmitted to the personal computer. This resultsin a message to the user displayed on the computer screen giving thelocation of the electrodes that are showing poor contact and requireattention In order to maintain a short signal about poor contact on theoutput of the circuit after the current pulse and to evaluate it by themicroprocessor, the triggers 20 are connected to the outputs of thecomparators. These triggers are set when the comparators come intoaction and are reset by the microprocessor before the next pair ofelectrodes are engaged and processed. As the triggers 20 made with CMOStechnology have a stable level of coming into action at about half ofthe supply voltage, they also can work as comparators. For the negativehalf-period overrun of the signal to register, it must be applied to oneof the triggers through an inverter.

The pulse phase modulator 21 is based on analog switches and provides a180° alteration of phase of the output signal of the excitation circuitonce during the period of measurements. Selected pairs of electrodes areconnected to the excitation circuit by two analog multiplexers 22 with acommon control address bus. The system of contact electrodes 1-16 isconnected to multiplexers 22 in such a way that a pair of activeelectrodes are always diametrically opposite electrodes (polarexcitation mode). Voltage from the receiving pairs of electrodes 1-16 ofthe contact systems is transmitted to the amplifier block of thetomograph via analog multiplexers 23.

The useful information is contained in the differential component of thevoltage measured between a pair of receiving electrodes. Differentialamplifier 24 must suppress the common-mode component of these voltages.Because it is impossible to achieve complete suppression of thecommon-mode component, we introduced a negative feedback loop withrespect to the common-mode component, based on an operational amplifier25. On the inverting input of this amplifier the common-mode componentfrom the pair of measured voltages is transmitted, and its output isconnected to the exciting pair via the modulator 21. The measureddifferential signal is superimposed by the contact potential difference,which amounts to ±300 mV for the stainless steel electrodes used in ourexperiments. Because this value may significantly exceed the dynamicrange of the amplifier (the useful signal amplitude amounts to tens orhundreds microvolts), the contact potential difference must becompensated. The function of measurement and storage of the values ofthe contact potential difference is performed by integrator 26 incombination with analog switch 27. The output voltage of integrator 26enters the zero correction input of the differential amplifier 24.Because the range of measured voltages is sufficiently large, weintroduced circuit 28 with controlled gain in order to initially reducethe dynamic range of the signal. Using this circuit, the total gain canbe controlled within three orders of magnitude by a code transmittedfrom the microprocessor 34 via a control bus.

The amplified voltage passes via a lock-in detector 29 and switch 30 tothe integrator 31. When the measuring cycle is completed, the voltage,accumulated at the output of integrator 31, allow us, taking intoaccount the gain of circuit 28, to determine the signal on the receivingpairs of electrodes. The signal from the integrator 31 enters ananalog-to-digital converter (ADC) 33, the output 12-bit binary data fromthe ADC are transmitted to the microprocessor 34. During the time of ADCoperation, integrator 31 is switched by switch 30 into the storageregime, and the initial state is set prior to the next measurement cycleby switch 32. This mode of connection of the lock-in detector andintegrator made the results of measurements less affected bylow-frequency or high-frequency noise components presented in thespectrum of the measured signal.

Operation of the analog circuit is controlled by logical signals of theS group supplied from the generator 35, and by the command groups Dproduced by the microprocessor unit 34. Generator 35 contains a counterof the clock pulses f_(T), and a read-only-memory (ROM) unit, and it isset into its cyclic operation regime by a logical level of signal Rsupplied from the microprocessor 34.

The microprocessor system controls a considerable number of operationsof the analog circuitry of the tomograph, provides a link between thetomograph and the personal computer (PC), and allows the user to modifythe tomograph's configuration.

For demonstration and analysis of reconstructed images, data are storedin archive. Images can be viewed one frame at a time or with successiveframes displayed continuously (“movie mode”). Individual frames can beincreased in size on the screen. Up to eight frames can be displayed onthe screen at once. Reconstructed frames can be spectrally analyzed whena series of measurements is obtained in automatic mode. In this case onecan see the spatial distribution of amplitude of definite frequencyharmonics in the static, one frame regime and the evolution of the imageat the same frequency, taking into consideration the phase ofoscillations in each mode, in the “movie mode”.

Measurements were performed on various locations of a human chest andextremities. The working electrodes were 30-mm diameter stainless steeldisks coated with the gel for electrocardiography. The electrodes werefixed with the help of an elastic belt. Some measurements were performedusing one-shot (expendable) electrodes of the Blue Sensor M-00-A type(Medicotest, Denmark). These electrodes provide lower contact resistanceand contact potential difference, but the data acquisition system of thetomograph and the reconstruction algorithm employed made the results ofvisualization sufficiently independent of the quality of contacts.

The advantages of the method presented here over previous inventions canbe shown quantitatively. If the value of noises is considered constantfor a concrete measuring device, the amplitude of signals and,therefore, signal-to-noise ratio depends strongly upon the techniquesused to acquire measurements. To illustrate, one can compare the ratioof minimum amplitudes of signals, measured on a pair of adjacentelectrodes, in the case of dipole current injection (for example, for asystem with sixteen electrodes, through electrodes 1 and 2—see FIG. 1),polar injection (for example, through electrodes 1 and 9) and injectionthrough two electrodes, between which two other electrodes are placed(for example, through electrodes 1 and 4) for a cylindrical object withhomogeneous conductivity distribution. In all cases, the injectingcurrent has the same value. In the case of dipole injection andinjection through separated electrodes, the minimum signal is measuredon pairs of electrodes which are diametrically opposite to the activepair (electrodes 9-10 and 10-11, correspondingly). In the case of polarinjection, the minimum signal is measured on four pairs of electrodesplaced symmetrically between two injecting (active)electrodes—electrodes 4-5, 5-6, 12-13 and 13-14. Assuming that the thanthe diameter of the studied object, and using elementary geometricalrelations, we obtain an estimation of the ratio of minimum amplitudes ofmeasured signals for polar _(p) and dipole _(d) injection:_(p)/V_(d)≈2{square root over (2)}D/d, where D—diameter of object (acircle on which electrodes are placed), and d—distance between adjacentelectrodes.

For the system with 16 electrodes, this ratio is 14.4. More precisecalculations give the coefficient 15.2 (a close ratio between theamplitudes of the measured signals is also obtained experimentally bymeasurements on a human thorax). When injection occurs through twoelectrodes, between which two other electrodes are placed, the minimumsignal is measured on the pair of electrodes most distant from theinjecting pair (electrodes 10-11). Calculations in this case show anincrease of 3.2 times the minimum amplitude of the signal, compared withdipole injection. Increasing of number of electrodes further increasesthe ratio of amplitudes of measured signals for non-dipole and dipoleinjection. Therefore, the use of polar injection in the system with 16electrodes produces an increased signal-to-noise ratio of more than anorder of magnitude, thus increasing the sensitivity and, in many cases,the resolution of the device.

To illustrate improvements in the polar injection tomography over dipoletomography, we present time dependencies of heart conductivity obtainedby using dipole injection (FIG. 2) and polar injection (FIG. 3). Polarinjection reliably shows changes of conductivity, caused by changes inthe blood filling the heart. This allows the researcher to determine thefrequency of heart's rhythm and to estimate the volume of blood ejectedby heart. Inages produced using dipole injection of current areconsiderably noisier. This makes determination of the frequency ofheart's contractions and the volume of blood more difficult. Theresulting measurements are not suitable for diagnostics.

Let's estimate quantitatively the accuracy of measurements and thequality of reconstructed images using the present invention, producedusing the correct choice of contact switching sequences, when the modeof current injection doesn't change. After switching the receiver fromone pair of contacts to another, a transient process occurs in the inputcircuits due to different values of galvanic potential differences onthe contacts. This transient process may not be completed by the timemeasurements of potential difference on electrodes have begun. This willcause errors due to the presence of a component on the operationfrequency of a tomograph in the spectrum of this process. The influenceof this process on the results of measurements is large, becausegalvanic voltage steps, occurring when electrodes are switched, amountto hundreds of millivolts, and they significantly exceed the minimumamplitude of measured signals by tens or hundreds microvolts. Theinfluence of transient processes can be decreased by increasing the timedelay between the moment of switching of a measuring pair of electrodesand the beginning of measurements. However, this will slow the operationof the system.

It is possible to reduce the influence of transients without reducingthe speed of device operation if during measurements the receiver isswitched from one pair of contacts to the other only after completion ofmeasurements with all possible pairs of injecting electrodes. Thetransient process is completed at the very beginning of the series ofmeasurements, and its influence is reduced significantly with the sametotal time required for completion of a full set of measurements. Forthe system with 16 electrodes and polar injection of current, the totalduration of measurements without switching measuring pairs of electrodesincreases 12 times using this mode of operation. The influence of thetransient process up to the moment of completion of measurements on agiven pair of electrodes is e¹²≈10⁵ times smaller.

BEST EMBODIMENT OF THE INVENTION

Example 1. FIG. 5 shows the reconstructed distribution of conductivityin a cross-section of the thorax of patient Y. Measurements were carriedout using polar injection of current. The spinal column 36, lungs 37,heart 38, and breastbone 39 are clearly visible. Large blood vessels canbe identified. The use of the adaptive algorithm of reference datasynthesis for image reconstruction makes it impossible to obtainquantitative information about conductivity directly from the results ofreconstruction. We normalized images, using the known values of theconductivity of bone tissues and blood (or close values of theconductivity of muscle along the fibers). During the normalization, apoint with minimum conductivity is determined on the image and ascribedbone tissue with a conductivity of 0.01 S/m. The point with maximumconductivity is ascribed a blood conductivity (0.5 S/m). The othervalues range between the two limits. The conductivity of lungs, obtainedupon this image calibration, agrees quite well with the results ofdirect measurements.

When a series of measurements is performed using the automated regime(with equal time intervals between measurements), obtained temporalvariations can be studied by use of Fourier analysis. FIG. 6 shows aspatial distribution of the amplitude of a spectral component ofconductivity with a frequency corresponding to the frequency of heartcontractions, calculated for the same series of measurements from whicha static frame is obtained. Spectral data processing has increased thecontrast of the regions of the heart, blood vessels and a portion of thelungs, where pulsations of conductivity are the most intense due tocirculation of blood.

FIG. 3 presents the time variation of conductivity for the point of theimage inside the heart (corresponding to heart's interior). This is animpedance cardiogram, which provides additional information about heartfunction. FIG. 4 shows a spectrum of this dependence. The first maximumcorresponds to the frequency of heart contractions; two other peakscorrespond to its harmonics. These characteristics open newpossibilities in cardiology because they contain parameters thatdirectly characterize the function of heart. They allow the researcherto estimate the volume of blood ejection by the heart, to determine thefrequency of the heart's rhythm, and to define more exactly bloodfilling in various parts of heart.

Example 2. Patient X has a diagnosis of central cancer of the rightlung, chronic sclerotic bronchitis. Roentgenography (FIG. 8) showed adecrease in area of the right lung, transparency of the upper part ofright lung is reduced, and the left lung is transparent. On theelectrical impedance tomogram (FIG. 7) one can distinctly see the changeof conductivity of the right lung—a virtual absence of air filling inthe measured cross-section. Light areas of the image in the area of theright lung indicate that conductivity of these areas is high because ofreplacement of low-conductivity lung tissue by dense tissue formations(primary tumor, metastasis and increased lymphatic nodes), and becauseof the presence of liquid (hydrothorax). The increase in the area andthe density of the image of the left lung points to an increase in airfilling the area to compensate for breathing inefficiency caused by thedisease. The results of electrical impedance tomography have goodcorrelation with Roentgenography data (FIG. 8) and reveal changes in thelungs more clearly.

Use of tomography for the diagnosis of various tumors offers thepotential for improved treatment of oncologic diseases and for thediagnosis of diseases of the mammary gland.

Example 3. FIGS. 9, 10 show the impedance tomogram of the lower andupper parts of the left leg of patient Z, where tibia 40, fibula 41,muscles 42, blood vessels 43 and femur 44 are clearly visible.Investigations revealed that it is possible to determine the changes ofconductivity of vessels due to varicose or thrombosis. This exampledemonstrates the potential to diagnose cardiovascular diseases anddiseases of bone tissuesoke-arthrosis, fractures and others. Changes inphysical loading leads to significant changes of muscle conductivity dueto changes of temperature and blood flow.

This invention can determine changes in the temperature of internalorgans, because Temperature changes are accompanied by change inconductivity of tissues. This could allow rapid identification ofinflammations due to infections and other conditions.

PRACTICAL APPLICATIONS

A rather simple, relatively inexpensive device, linked with a standardpersonal computer, to create tomographic images based on the electricalimpedance of an object can be made. Measurements are safe for patientand attendants. The conductivity distribution reconstruction algorithmallows one to visualize high quality, informative images with the helpof a tomograph familiar in look and presentation in the field ofmedicine. Results obtained by reconstruction of absolute conductivity invivo show that electrical impedance tomography can be widely used inmedical practice and clinical investigations.

The method presented here can also be used to reveal and measurestructures and processes. Visualization of the conductivity of tissuesmakes it possible to observe the processes of internal hemorrhages andinflammatory processes. The distinctions in conductivity of fat tissuesand muscles will allow the researcher to define the distribution ofmuscle tissues. The specific conductivity of lungs at inhalation andexhalation differs three times because of changes of air filling thelungs; thus emphysema diagnosis is possible. Appreciable changes intissue conductivity occur as a result of necrosis and changes in thedensity of tissues, improving the study of the behavior of tumors duringtreatment of cancer patients.

In comparison with x-ray or NMR tomography electrical impedancetomography is more rapid, it allows the researcher to visualize thechanges of conductivity during one cardiocycle and to observe bloodfilling the heart and vessels. Inflammations and some other pathologicalprocesses are accompanied by an increase in the temperature of tissues.The tomograph allows one to determine the temperature of interior organsand to diagnose many diseases at an earlier stage.

The high sensitivity of the method to variations of physiologicalcondition of tissues and organs, good contrast in the obtained images,high rate of measurements, safety for attendants and patient, low costof the device and simplicity of operation indicate the invention couldhave wide application in medical practice.

We claim:
 1. A method for the construction of images of a body usingelectrical impedance tomography, comprising: a) placing a series ofcontact electrodes at spaced intervals on the surface of a body; b)selecting a pair of electrodes for a set of potential differencemeasurements, each measurement made by connecting an electric currentsource to pairs of non-selected electrodes sequentially, said pairs ofelectrically connected electrodes being separated by at least two otherelectrodes; c) selecting different pairs of measurement electrodes andrepeating step b) for each pair until all pairs of measurementelectrodes had been selected; d) determining the reference values of thepotential differences u_(r)^(i)(j)

by the approximation of the measured distribution of the potentialdifferences u_(m)^(i)(j)

in accordance with the expressionu_(r)^(i)(j) = c₁^(i)f₁^(i)(j) + c₂^(i)f₂^(i)(j) + c₃^(i)

while the electric current source is connected to the i^(th) pair ofelectrodes, and where i is the number of the exciting pair ofelectrodes, j is the number of the measuring pair of electrodes,f₁^(i)(j)

is the given distribution of voltage between adjacent electrodes alongthe boundary of the reference object, f₂^(i)(j)

are signals caused by spurious couplings, and c_(α)^(i)(α = 1, 2, 3)

are the approximating coefficients of the measured distribution of thepotential differences; and e) reconstructing the image of the spatialdistribution of conductivity of said body by back projection alongequipotential lines according to the expressions:$S = {\sum{\frac{W^{lt}W^{rt}}{W^{lt} + W^{rt}}( {\lambda^{lt} + \lambda^{rt}} )}}$

λ^(lt, rt) = u_(r)^(lt, rt)/u_(m)^(lt, rt) − 1

where W^(lt), W^(rt) are weighting factors determined according to theprocedure of back projection in the direction from the left and from theright intersection of the equipotential line with the surface of saidbody, S is summed over all positions of the injecting electrodes,u_(m)^(lt, rt)

are voltages measured on the left and on the right ends of theequipotential line that passes through the given point of thereconstructed cross section, and u_(r)^(lt, rt)

are the reference potential differences corresponding to a body withhomogeneous conductivity or determined according to claim 1 d.
 2. Themethod of claim 1, wherein said electric current source is connected toa pair of diametrically opposite electrodes.
 3. The method of claim 1,wherein said electric current source is produced by a pulsed voltage inthe form of two periods of meander, shifted by 180 degrees in phase witha controlled amplitude that is established by an 8-bit code.
 4. Themethod of claim 1, wherein the signals caused by spurious couplings aredetermined by measurement.
 5. The method of claim 1, wherein a series ofelectrical impedance tomographs are produced sequentially in time of abody in which the conductivity varies in time and a spectral Fouriertransformation of the time dependencies is obtained, thereby permittingthe reconstruction of the images of spatial distribution of conductivityof the body for each frequency component.
 6. The method of claim 1,wherein the reconstruction of the image of the spatial distribution ofabsolute conductivity of a body is carried out by normalizing theobtained conductivity values on the basis that the least value ofconductivity corresponds to the conductivity of bone tissues and thegreatest value of conductivity corresponds to the conductivity of blood.7. An electrical impedance tomograph device, comprising: a. a pluralityof contact electrodes positioned adjacent to the surface of a body atspaced intervals; b. current source means to inject current into a pairof excited electrodes separated by at least two other electrodes; c.means to measure potential differences in other electrodes pairs inducedby said pair of excited electrodes such that measurements can be made ona given pair of electrodes while said current source is connectedsequentially to each pair of the remaining electrodes and thismeasurement procedure is repeated on each of the remaining electrodepairs; d. circuit means to compensate for the contact potentialdifferences on each electrode; e. computer means to determine thereference values of the potential differences u_(r)^(i)(j)

by the approximation of the measured distribution of the potentialdifferences u_(m)^(i)(j)

in accordance with the expressionu_(r)^(i)(j) = c₁^(i)f₁^(i)(j) + c₂^(i)f₂^(i)(j) + c₃^(i)

while the electric current source is connected to the i^(th) pair ofelectrodes, and where i is the number of the exciting pair ofelectrodes, j is the number of the measuring pair of electrodes,f₁^(i)(j)

is the given distribution of voltage between adjacent electrodes alongthe boundary of the reference object, f₂^(i)(j)

are signals caused by spurious couplings, and c_(α)^(i)  (α = 1, 2, 3)

are the approximating coefficients of the measured distribution of thepotential differences; and; f. computer means to reconstruct the imageof the spatial distribution of conductivity of said body by backprojection along equipotential lines according to the expressions:$S = {\sum{\frac{W^{lt}W^{rt}}{W^{lt} + W^{rt}}( {\lambda^{lt} + \lambda^{rt}} )}}$

λ^(lt, rt) = u_(r)^(lt, rt)/u_(m)^(lt, rt) − 1

where W^(lt), W^(rt) are weighting factors determined according to theprocedure of back projection in the direction from the left and from theright intersection of the equipotential line with the surface of saidbody, S is summed over all positions of the injecting electrodes,u_(m)^(lt, rt)

are voltages measured on the left and on the right ends of theequipotential line that passes through the given point of thereconstructed cross section, and u_(r)^(lt, rt)

are the reference potential differences corresponding to a body withhomogeneous conductivity.
 8. The device of claim 7, wherein said currentsource means is connected to a pair of diametrically oppositeelectrodes.
 9. The device of claim 7, wherein said current source meansis produced by a pulsed voltage in the form of two periods of meander,shifted by 180 degrees in phase with a controlled amplitude that isestablished by an 8-bit code.
 10. The device of claim 7, furtherincluding a comparator means by which voltages measured at otherelectrodes that fall outside an allowable range are flagged forattention.